The production of sufficient quantities of magnesium oxide to meet the ever-growing commerical demands has been hindered by a world-wide shortage of magnesium sources. Even in instances where magnesium sources are available for processing, the competition for available calcining facilities necessary in the production of various grades of magnesium oxide (magnesia), for example, light-burned magnesia and dead-burned magnesia, is severe.
The particular grade of magnesium oxide is essentially a function of the manner in which the magnesium oxide is produced. Generally, magnesium oxide is produced by the thermal decomposition of various magnesium compounds such as the magnesium salts of volatile acids. Commercial grades of magnesium oxide are generally prepared by the thermal decomposition of magnesite (MgCO.sub.3), magnesium hydroxide or basic magnesium carbonate. The hydroxide and carbonate can be decomposed to the oxide at relatively low temperature (e.g., about 700.degree. to about 1650.degree. F.) and the resulting oxide is generally moderately active (which is understood by those skilled in the art, is a measure of the degree of acid-reactivity of the oxide) and highly available (i.e., above about 90% by weight or more of the magnesium oxide is soluble in dilute, e.g., about 4 weight percent, hydrochloric acid). This low decomposition temperature-produced magnesium oxide is known generally as "light-burned" magnesia.
To the contrary, oxide prepared from higher temperature decomposition of the carbonate (e.g., about 2500.degree. to about 3600.degree. F.) or light-burned magnesia which is further calcined at such relatively high temperatures has essentially no activity and is generally unavailable (i.e., the weight percent of magnesium oxide being soluble in dilute hydrochloric acid is well below the 90% by weight level). This relatively dense, hard highly refractory product is generally known as "dead-burned" magnesia and is generally inactive and unavailable even when ground into fine particles and independent of its activity and availability prior to calcining. Thus, even the low decomposition temperature-produced magnesium oxide (light-burned magnesia) which is relatively highly available may be made unavailable by further high temperature calcination to the dead-burned form.
It will be understood by those skilled in the art that magnesia having moderate activity is useful in a variety of circumstances where magnesia having essentially no activity (e.g., dead-burned magnesia) is not usable.
A substantial area of such use for light-burned (moderately active) magnesia is in conjunction with a variety of pollution control processes. Moderately active magnesia may be incorporated as a fuel additive in oil-fired utility power station furnaces to reduce the buildup of acid smut, thus increasing the desirability of employing oil as the fuel source for the production of electrical power and is also useful in stack-gas scrubbing processes where coal is chosen as the fuel for the production of electrical power. Another growing source for the use of light-burned magnesia is water treatment processes, where the magnesia is used to neutralize undersirable acids.
Light-burned magnesia is also useful in the agricultural area, and is incorporated in both fertilizer and animal feeds.
Magnesium is recognized as a necessary adjunct to the diet of animals, poultry, swine and the like. For example, adult cattle and sheep feeding on rapidly growing young grass in the spring or calves feeding entirely on milk (milk being low in magnesium) are susceptible to being affected with grass tetany, a disease characterized by hypomagnesemia. Tetany causes nervousness and an awkward gait followed by muscle tremors, rapid breathing and collapse of the animal. Similar symptoms have been noted in horses and poultry.
While treatment is frequently successful, prevention of such a condition would be preferable. Prevention is generally accomplished by adding magnesium to the diet. Although the magnesium supplement may be added in the form of various salts (e.g., magnesium citrate, acetate, nitrate, lactate, chloride, trisilicate or sulfate), it is generally added as magnesium oxide (magnesia). See, e.g., The Merck Veterinary Manual, Merck and Co., Inc., 1967, pages 556-558.
Magnesium oxide is commercially available, for example, as a food supplement for beef and dairy cattle, horses, sheep, swine and poultry and for use in dog and cat food in the form of particles (the majority of the particles having a size generally in the range of -20 +200 mesh) containing about 90% by weight or more light-burned, highly soluble magnesium oxide (and concomitantly 50% by weight or more magnesium). The insoluble portion of the magnesium oxide essentially passes through the animal or poultry and remains inactive.
Light-burned magnesia is particularly useful in pulp and paper processing. It is employed as a decolorizing agent whereby the particular color of the final paper product may be controlled. Light-burned magnesia is also useful as a constitutent in various formulations for paper fillers and coatings.
Other applications for light-burned magnesia include use in sugar processing, production of oxychloride and oxysulfate cements, rayon and rubber production, production of electrical heating rods and as an asbestos substitute in fireproofing. Further, the light-burned magnesia may be employed as a boiler cleaning agent and may be substituted for conventional neutralizers such as caustic soda, soda ash and sodium bicarbonate. In substantially all of the known uses for light-burned magnesium oxide, the magnesium oxide must have a color which is white or nearly white (e.g., buff-colored) and low carbon content. Dark-colored magnesium oxide (as from impurities or additives) or magnesium oxide containing appreciable amounts of carbon are generally not commercially acceptable for such uses.